I just can say that an italian telecom company, Eutelia, uses the Mediatrix fxs to sip gateway, to connect phones and faxes to their sip network (they use Asterisk too); you could try the Audiocodes bri to sip gateways, I used their PRI, fxo and fxs sip gateways in the past and they perfomed well in the vast majority of cases, just some problems with faxes over ip.