Okay... it might just be me, but when the brothers MacManus decide to give Agent Smecker a ring, they look at his card and dial him up on a pay phone... and he answers on a pay phone... either the FBI is slipping on their funding, or someone wanted to hurry the scene through post...
I'm going to assume that the number that they called from the card was either a pager number or forwarded to Smecker's pager... just a guess though, and Smecker called the payphone number that the McManus brothers were using.
The DEA and the FBI both have cell phones that are distributed to its 'employees' through the government. Therefore, if the Detective had the Saints call him on his cell, it would've been easily traced, given that VigilanteISM isn't exactly LEGAL, although Smecker agreed with what the boys were doing....