It depends on the facility and how their phone approval system works, and the answer is different in federal versus state and county systems.
In the federal system, inmates manage their own approved call lists and add numbers directly through the phone system at their unit. There is no automatic flag that cross-references approved numbers against co-defendant status. Unless a specific no-contact order has been issued by the court prohibiting communication between co-defendants, a federal inmate can technically add your number and call you without the system blocking it.
In state prisons and county jails, the process varies significantly. Some facilities require submitted phone lists that go through a review and approval process before any number becomes active. If that facility screens for co-defendant relationships during that review, your number could be flagged and denied. Other facilities handle it the same way federal does, with the inmate managing their own list without a co-defendant cross-check.
The more important question is whether there is a court-ordered no-contact provision between you and your co-defendant. Judges routinely impose no-contact orders between co-defendants as a condition of pretrial release or as part of a plea agreement, specifically to prevent witnesses from coordinating stories or interfering with the case. If such an order exists, it applies regardless of what the phone system does or does not catch. Violating a no-contact order from either end creates serious new legal problems for both of you.
Check the court paperwork carefully before any calls happen. If there is no court order prohibiting contact and the facility's system allows it, the call can go through. But confirming there is no order is the essential first step.