There are two ways to handle this:
1. The weapon acts as the catalyst(s) for the spell
This is the more powerful option of the two options, as it means there are no requirements to using the ability. I personally was granted a similar weapon - my Greatsword was able to cast Earth Tremor 3 times a day. If you use this method, then no, there is no additional attack damage.
I would suggest that if you go with this option, you amend it to 3 uses per day, to avoid it getting abused by simply trying to take a long rest every time you run out of charges.
2. Option to cast it on hit
There are some weapons/skills/spells that already have this kind of ability (and I again, have been granted a similar ability in a game). The requirement to first hit the target, in order to be able to cast the spell as additional damage, works as a strong nerf to this ability. Firstly, if you miss, you lose the ability to cast the spell. And secondly, if you deal a solid amount of damage to the target, you might not want to "waste" a spell slot on a measly orc, therefore limiting the usage of this ability to a more "worthy" target. If you use this method, you deal your attack damage, then deal the Call Lightning damage. These two are separate abilities.
Honestly, there are a few factors that this spell requires in order to cast.
Firstly, Concentration, up to 10 minutes. As part of the spell ability, if you maintain concentration on this spell, you can automatically cause damage if you use your action.
On each of your turns until the spell ends, you can use your action to call down lightning in this way again, targeting the same point or a different one.
This means that you can basically call the spell once, then spend the rest of the battle just continuing to cause damage with the spell (in combat, 10 minutes is 100 rounds). Additionally, regardless of whether or not you add your base attack damage to the spell damage, it would only occur on the first round of Call Lightning.
Secondly, this is an AoE. This effectively means that you would either need immunity to the spell, or be forced to make a Dex check every time you cast the spell if this was to be cast on a hit (based on the assumption that the spell is cast on the point of contact, when you cast it on a hit).
A bolt of lightning flashes down from the cloud to that point. Each creature within 5 feet of that point must make a Dexterity saving throw. A creature takes 3d10 lightning damage on a failed save, or half as much damage on a successful one.
Finally, the primary limitation of the spell is the accommodation of the storm cloud.
A storm cloud appears in the shape of a cylinder that is 10 feet tall with a 60-foot radius, centered on a point you can see 100 feet directly above you. The spell fails if you can’t see a point in the air where the storm cloud could appear (for example, if you are in a room that can’t accommodate the cloud).
So if you're casting this in a dungeon or a cellar, it will automatically fail.
Ultimately, I would recommend a couple of things (based on the assumption that you probably already haven't - if you have, no worries):
- Keep the attack damage and the Call Lightning spell damage separate, to remove confusion, and make the spell casting optional (i.e. if you hit, you choose whether or not to cast the spell)
- Remove Concentration. Each "cast" only goes off once (3d10 is a huge amount of damage).
- Don't apply any modifiers to the Call Lightning Damage. As I have stated above, the attack damage and Call lightning damage should be kept separate. And following the description of the spell, there are no modifiers to the damage of the spell. Only casting it, which is based off your spell casting ability.
- I'm honestly 50/50 on this, but potentially remove the requirement for the cloud to use this ability. Sine it's only going to be used once a round, to a maximum of three times at most.