acejavelin, hopefully I'm answering your question. We are not on SIP and don't have analog lines. We have a smart voice PRI with Telepacific. Telepacific is then piggy-backing off Time Warner Cable's fiber line.
Gotcha, this is most likely what we call a "SIP PRI", meaning it is SIP into a gateway device, such as an Adtran unit, but it is converted to a PRI to be presented to equipment as a digital trunk. Works fine as long as your internet connection is solid.
Lag and call quality issues are often the result of your carrier though... if it is reproducable test it. Is there lag/delay from phone to phone internally, or just when involving and outside party? My guess is it is just when an outside call is party, and it is exasperated when it is a call in and out. Get call examples with date, time, duration, issue, originating, and terminating number and call your carrier and report it. If the calls are bad internally, that is a while different ball game, probably internal LAN network QOS issues of some kind.