Any particular reason for not disclosing the model of cordless phone or SMC router?
Several 5GHz cordless phones have the base unit transmit on 2.4GHz. Such cross band devices also exist with 2.4/900MHz such as the Panasonic Giga-Range. Your SMC is probably getting trashed only by the 2.4Ghz from the base unit. If it's a frequency hopping spread spectrum (FHSS) base, then moving to a different channel on the SMC will not help.