This seems to be a hot topic in a few places. Honestly it is simple if you read the descriptions in the source for the effective modification.
There are Three different types of ECL that can effect a caster: Effective Character Level bonus, Effective caster level after determining Spells per day and known, Effective Caster Level before determining spells per day, known and caster level.
+ Effective Character Level - This is exactly what it says, you have a boon, effect or PRC that is granting you the next step on the class table, bab, saves, specials, spells per day, known and power level(caster level).
+ Effective caster level after determining Spells per day and known - This increase the power, durations, effects, DC of spells, but does not grant you additional spells or higher level spells. This would be the common effect of a ring or amulet or item, although some other things like feats, and PRCs may do this also.
+ Effective Caster Level before determining spells per day, known and caster level - This is common with PRC class benefits. You do not gain the BAB, Saves, or specials for your caster class. You do gain level of spells you can cast, number of spells per day and spells known at the appropriate effective caster level, and casting level increase. An example of this is the Mystic Theurge, ref pp387,388 Pathfinder Core Rulebook,
"adds the level of mystic theurge to the level of whatever other
arcane spellcasting class and divine spellcasting class the character
has, then determines spells per day, spells known, and caster level
The text is clear. There are a few PRCs that make exception to this, but for the most part those are Homebrew PRCs, but as a rule this is how the "+1 level of/to existing" works in any PRC.
This said, remember your GM/DM has overriding privileges in the running of their world, so if they give you a house rule that is disparaging to books and guidelines, it is their world, try and keep ruling debates off the table as more private discussions. Do not lose gaming time for you or others to debate rules. ~~~~