I'm not giving 'FAKE NEWS'.
"realfoneguy" is superficial, in this case.
You mentioned KX-TA30874 card, which intendent for European and Worldwide models. If Your system has a serial RS-232 port, and You are from market which use 220V AC power network, it can increase capacity.
KX-TA624 (USA, UK) use KX-TA62470 and the KX-TA62477 Expansion Cards.
KX-TA308 Australia, Europa etc. (with serial port) 220V AC power is older model (generation) and it's upgradeable with KX-TA30874 and KX-TA30877 cards.
KX-TA308 USA (with serial port an built-in CID card) 110V AC power, has different oriented text on case, and it is not upgradeable.
System KX-TA308 with serial port is also labeled as KX-TA616 and KX-TA624 in most countries.
This label is for previously prepared (selling) configuration/capacity, because you can reduce capacity of KX-TA624 to 3/8 or increase KX-TA616 to KX-TA624 capacity.
So, in Europa, KX-TA308 is same phone system as KX-TA624 in UK and USA and can be upgradable to 6/24. Every system from same older generation KX-TA624, KX-TA616, and KX-TA308 has a serial RS-232 port, so You can easy recognize generation, and for markets other than USA (110 V AC), always upgradeable.
Next, newer generation very similiar by outside look, use USB port, and different labels in areas of sale, KX-TEA308 and KX-TA824 (intended for USA market) is almost same systems as KX-TEA308. KX-TEB308, and KX-TES/TEM 824 in other countries.
KX-TA824 (using KX-TA82470, KX-TA82471, KX-TA82474 Expansion Cards) system labeled in USA is KX-TES/TEM 824 in other countries.
Conclusion;
If you have any KX-TA308, 220V AC with serial programming port, You can increase capacity to max 6 lines, 24 extensions, with two types of expansions card. If You have KX-TA308 USA, You can't upgrade.
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If you respond with the numbers on the model plate and your country of origin, you have a better chance at an correct answer.[/highlight]