Over the past two decades, mobile phone ownership has surged, integrating into our daily lives and personal spaces. Smartphones, with their flexible user interfaces, enable quick online communication, efficient task navigation, and easy access to external networks. This constant connectivity often prompts users to pick up their phones around others. Research on this phenomenon, often termed “phubbing” or “technoference,” has focused on the relational harm caused by one partner using their phone in the presence of the other. Studies in intimate relationships have linked phubbing to lower relationship quality, reduced intimacy, and increased conflict over technology use. However, there are three major shortcomings in the existing literature. First, the cross-sectional designs used so far do not allow for conclusions about daily processes, limiting the understanding of phubbing's daily impact on relationships. Secondly, studies have focused on relational consequences but overlooked the interactions where co-present mobile phone use occurs, relying mainly on existing theoretical frameworks to understand immediate interaction impacts. Thirdly, as the field evolves, it needs continuous scrutiny and evaluation of its conceptual and methodological components to advance psychological science. Issues such as suboptimal conceptualisations and measurements in existing research need to be addressed. This thesis aims to expand the understanding of co-present phone use in relationships while addressing existing shortcomings. Structurally, it consists of three blocks of studies and publications, each containing chapters that address specific research limitations. The following research demonstrates that co-present phone use is a highly complex phenomenon requiring a multimodal approach. This approach combines individuals' subjective reports (assessments, experiential perspectives, etc.) and tools to capture objective measures (e.g., behavioural patterns, screen time) to understand its potential impact on relationships. This dissertation emphasises that co-present phone use is not inherently harmful, contrary to the dominant paradigm. Understanding its role in one's life and relationships should be integrated with other person-specific factors characterising daily functioning. The thesis offers potential avenues to be explored in future research to advance the field and psychological science.