Is it permissible to continue telephone communication with a former girlfriend, after the marriage of both of them, merely to check on her and inquire about her well-being, while feeling psychological comfort from this, or must this relationship be severed?

Marriage should not be prevented due to tribal considerations. The Prophet, peace and blessings be upon him, established the criteria for choosing a wife based on religiosity, wealth, lineage, and beauty, and did not mention tribe. The temptation of women is among the most dangerous temptations, and a relationship with a foreign woman is restricted by strict controls. As for a relationship with a married woman, it is an even greater sin and is considered corrupting a woman against her husband, which is a grave offense. What you imagine to be the innocence of this relationship is a deception from Satan, and this contact must be severed immediately.