This inexpensive
AIEK Q1 Ultra-thin Card Phone, US $13.99 + FREE SHIPPING to US Via Unregistered Air Mail, seems to have 1-core CPU with 128 MB RAM. The least expensive I found is from this
eBay seller (but a different model C6). Too bad, it does not have a WiFi capability. Otherwise, it may be a target for anyone here is interested to hack / run any Linux OS and make it compatible to any SIP phone.